1. Field of the Invention
The present invention relates to an imaging optical system for compact cameras.
2. Description of the Related Art
Digital cameras or video cameras that have solid state pickup devices, such as charge coupled devices (CCDs) or complementary metal-oxide semiconductors (CMOSs), are widely in use. In particular, mega-pixel camera modules are in demand, and cameras having at least 5 million pixels and providing high definition are being developed. Imaging optical apparatuses, such as digital cameras or cellular phone cameras that use pickup devices such as CCDs or CMOSs, are required to be compact, light, and cost-effective. Moreover, cameras in mobile phones should be compact in order to be used in image communications. In order to obtain compact and light cameras, an imaging optical system made up of three lenses is used.
When a camera module, included in a mobile apparatus for image communications, is comprised of three plastic lenses, ¼″ camera modules are mainly used. When an imaging sensor is reduced in size, ¼″ camera modules are mainly being used. In order to solve shading caused by the reduction in size of image sensors, as much ambient light as possible needs to be secured, and the telecentric characteristics of the lens system need to be set high. However, in a conventional case where an aperture stop is installed between the first lens and second lens, the telecentric characteristics are degraded, and a restrictive condition that an injection should be performed with a rib thickness, which deviates from an effective curvature, set to be less than or equal to 0.4 mm with a decrease in an overall height of a lens needs to be satisfied. This restrictive condition makes it difficult to mass produce. In addition, in a conventional case where a 0.3 mm infrared filter is used between a lens and an image sensor, the infrared filter should secure a long back focal length of about 1.1 mm. In order to meet these requirements, a precise lens designing technology for maintaining a high optical performance by appropriately arranging the refractive powers of lenses that constitute a photographing lens is needed.